1. Field of the Invention
The present invention relates to a semiconductor device, and a method of manufacturing the same.
2. Description of Related Art
A liquid crystal display device (LCD), which is one of thin type panels, has been broadly used as a monitor for a personal computer, that for a portable information terminal device, that for an in-vehicle monitor such as a car navigation system, and the like, while taking advantage of low power consumption, small size, and lightweight. Furthermore, in recent years, the liquid crystal display device has been widely used in a TV set, and will replace a conventional cathode-ray tube. Further, an organic EL (Electro-Luminescence) display device, which overcomes the problems of an LCD such as restrictions of view angle and contrast and difficulty of following ability of high-speed response to a motion picture, has been employed as a next-generation thin panel device. The organic EL display device has features which an LCD does not have, such as self-luminous type, wide view angle, high contrast, and high-speed response.
Thin film transistors (hereinafter referred to as TFT) used for such a display device often has a MOS structure using a semiconductor film. As TFTs, there are several kinds including the inverse staggered type (bottom-gate type) and the top-gate type. Further, semiconductor films often include an amorphous silicon (a-Si) film, which is an amorphous semiconductor film.
In the TFT that uses the a-Si film as the semiconductor film, trapping and injection of electrons from the a-Si film to the gate insulating film and the localized level density in the a-Si film increase. Thus, such a TFT has a drawback in that the shift of the threshold voltage occurs. In order to compensate for this drawback, the circuit design is made in consideration of the shift amount of the threshold voltage in advance. However, the TFT using the a-Si film can only be used as a TFT for switching a pixel portion, and cannot be used in a gate driver circuit or the like. In order to deal with this, a gate driver IC is externally provided in the display device for gate driver. Accordingly, the frame of the display device inevitably increases.
In order to solve the above problem, the gate driver circuit also needs to be manufactured by the TFT as well. Thus, crystalline semiconductor films such as a microcrystalline semiconductor film, a polycrystalline semiconductor film and the like are used as the semiconductor film. The crystalline semiconductor film has smaller defect level density compared with the amorphous semiconductor film. Accordingly, the TFT that uses the crystalline semiconductor film does not cause the shift of the threshold voltage, or causes only a small amount of shift, if any. In recent years, especially, a semiconductor film of a stacked layer structure has been proposed that forms the microcrystalline semiconductor film in the semiconductor film that contacts with a gate insulating film, and the amorphous semiconductor film on the microcrystalline semiconductor film.
As a method of forming the microcrystalline semiconductor film, a deposition method by a plasma CVD (Chemical Vapor Deposition) method has been known (see for example, Japanese Unexamined Patent Application Publication No. 2005-167051). Further, a method has been known in which an amorphous semiconductor film is first formed, and thereafter the amorphous semiconductor film is irradiated with a laser beam, so as to microcrystallize the semiconductor film (see for example, Japanese Unexamined Patent Application Publication Nos. 2005-167051, 2007-5508, and 2007-35964).
However, in the microcrystalline semiconductor film that is formed by the deposition method by the plasma CVD method, an incubation layer is formed in an initial stage of forming a film. In summary, the amorphous semiconductor film before starting the crystal growth is formed in an initial stage of forming a film. Thus, the semiconductor film that contacts with the gate insulating film is an amorphous semiconductor film. As such, trapping and injection of the electrons from the amorphous semiconductor film to the gate insulating film and the localized level density in the amorphous semiconductor film are increased. Then, the shift of the threshold voltage occurs. Further, according to the method in which an amorphous semiconductor film is first formed, and thereafter the amorphous semiconductor film is irradiated with a laser beam, so as to microcrystallize the semiconductor film, the semiconductor film that contacts with the gate insulating film is a microcrystalline semiconductor film. The microcrystalline semiconductor film has a small defect level density. Thus, the TFT using the microcrystalline semiconductor film that is formed according to the method above does not cause the shift of the threshold voltage, or only causes small amount of shift, if any.
However, according to the method in which an amorphous semiconductor film is first formed, and thereafter the amorphous semiconductor film is irradiated with a laser beam, so as to microcrystallize the semiconductor film, a native oxide film is formed on the surface of the microcrystalline semiconductor film that is formed. Further, as the microcrystalline semiconductor film is exposed to an atmosphere, the microcrystalline semiconductor film is contaminated. If the amorphous semiconductor film is formed on the microcrystalline semiconductor film to manufacture the TFT with this state, the initial characteristics degrade. More specifically, the on current decreases and the off current increases. Then, in order to remove the native oxide film on the surface of the microcrystalline semiconductor film and to clean the surface of the microcrystalline semiconductor film, the hydrofluoric acid processing is performed after forming the microcrystalline semiconductor film. If the amorphous semiconductor film is formed after that, the initial characteristics are improved.
Furthermore, a method of separating the use of the TFT using the amorphous semiconductor film and the TFT using the polycrystalline semiconductor film as the semiconductor film according to the roles of TFTs has also been proposed (see for example, Japanese Unexamined Patent Application Publication No. 5-55570). However, according to this method, the shift of the threshold voltage occurs with the TFT using the amorphous semiconductor film.
Furthermore, a method of providing a low resistance semiconductor film or a metallic film between the gate insulating film and the amorphous semiconductor film which is the semiconductor film has also been proposed (see for example, Japanese Unexamined Patent Application Publication No. 5-190857). However, the low resistance semiconductor film or the metallic film only functions as the low resistance element, and does not function as a TFT channel with this structure. In summary, it is the amorphous semiconductor film that functions as a channel, and thus, the shift of the threshold voltage occurs.
Now, with reference to FIG. 7, a manufacturing method of a semiconductor device with a microcrystalline silicon TFT according to a related art will be described. FIG. 7 is a cross sectional view showing the structure of the semiconductor device with the microcrystalline silicon TFT according to the related art.
First, a metallic film is formed on a glass substrate 200 by sputtering. An alloy material or a metal material such as Al, Cr, Mo, Ti, W or the like may be used as the metallic film. Then, a resist pattern is formed on the metallic film by photolithography. After that, the metallic film is patterned into a desired shape with an etchant. Then, the resist is removed. A gate electrode 201 is thus formed.
Next, a gate insulating film 202 is formed by a CVD method. Then, an a-Si film which is an amorphous semiconductor film is formed by the CVD method. Next, the concentration of hydrogen in the a-Si film is decreased by an annealing process. Then, the a-Si film is crystallized by laser annealing, so as to form a microcrystalline silicon film 203 which is the microcrystalline semiconductor film. Next, hydrofluoric acid processing is performed to remove the oxide film formed on the surface of the microcrystalline silicon film 203 and to clean the surface of the microcrystalline silicon film 203. Then, an a-Si film 204 which is the amorphous semiconductor film and an n-type amorphous silicon (n-a-Si) film 205 which is the n-type amorphous semiconductor film are formed by the CVD method.
Next, a resist pattern is formed on the n-a-Si film 205 by photolithography. After that, the n-a-Si film 205, the a-Si film 204, and the microcrystalline silicon film 203 are collectively patterned into a desired shape with the etchant. Then, the resist is removed.
Next, a metallic film is formed on the n-a-Si film 205 by sputtering. An alloy material or a metal material such as Al, Cr, Mo, Ti, W or the like can be used as the metallic film. Then, a resist pattern is formed on the metallic film by photolithography. After that, the metallic film is patterned into a desired shape with an etchant. Thus, a source electrode 207 and a drain electrode 208 are formed.
Next, the whole n-a-Si film 205 and a part of the a-Si film 204 corresponding to the channel region are removed by etching while leaving the resist on the metallic film. A back channel etch portion 206 is thus formed. Next, an SiN film 209 which is a protective film is formed by the CVD method. Then, the resist pattern is formed on the SiN film 209 by photolithography. After that, the SiN film 209 is patterned into a desired shape by etching, so as to remove the resist. As such, a contact hole 210 that reaches the drain electrode 208 is opened.
Next, an ITO film, which is a transparent conductive film, is formed by sputtering. Then, the resist pattern is formed on the ITO film by photolithography. After that, the ITO film is patterned into a desired shape with the etchant. Then, the resist is removed. A pixel electrode 211 is thus formed. According to the above process, the semiconductor device having a microcrystalline silicon TFT structure is completed.
However, when the amorphous semiconductor film is formed on the surface of the microcrystalline semiconductor film after the hydrofluoric acid processing is performed as described above, a film floating or a film peeling occurs between the microcrystalline semiconductor film and the amorphous semiconductor film due to the film stress of the amorphous semiconductor film or poor adhesion of the microcrystalline semiconductor film with the amorphous semiconductor film. The condition in which the film floating or the film peeling occurs is not always the same, but it varies depending on the situations. Some TFTs have good initial characteristics, which others have poor initial characteristics. This decreases the yield rate, and accordingly, decreases the production efficiency as well.